My idea for an SCP would be this purple space-like entity in the shape of the human with a 10 cm iron ball inside it. Its anomalous property is that much like a sun it gets energy from the fusion of hydrogen atoms in its body that it absorbs from the air. With this energy it can then explode and implode any object it touches with the energy it has at the moment. The entity can also explode and implode itself and reforms itself at the iron ball which is unharmed. Containment would include trying to make its chamber as close as possible to a vacuum and would include extremely powerful magnets to repel the entity into a fix position.

Not sure about this last part but an idea I would have for the SCP is for it to be saying that the Foundation "Left them to die" and therefore having a vendetta against the Foundation and wishes to destroy it. The SCP would also want to "Free the others." As I said before I'm not sure about this part but if it needed some lore behind it this would be my idea.

I was a little disappointed when I read that this one was a humanoid SCP idea. I personally feel like a living supernova observable in outer space would be a lot more interesting and probably has a more developed story behind it.

As for the concept you have, it's pretty weak. I'm no physicist, but I do know that whenever you split or fuse an atom, there's a nuclear explosion. Does that mean that this thing nukes everything around it? How would the Foundation have even the slightest chance of containing it? And why is its core an iron ball that's specifically 10cm?

Also, it's a humanoid, which are really tough to write well, and this one feels pretty weak in concept. I highly recommend you take a look at the Humanoid tab of the How to Write an SCP guide if you haven't already. I'd also recommend asking some veteran members with experience writing successful humanoid articles for some tips and tricks.

The last part of the idea doesn't really do it for me either. Unless you give a really strong backstory for this thing and why it would have a vendetta against the Foundation; which would probably be better saved for something like a Tale.